The present invention relates to an optical film and in more detail to an optical film which exhibits low hygroscopicity, high transparency, high heat resistance, and markedly lowered brittleness via blending of specified acrylic resins and cellulose ester resins.
Demand for liquid crystal display devices is increasing in applications to liquid crystal TV sets and liquid crystal displays of personal computers. Commonly, a liquid crystal display device is composed of a liquid crystal cell in which a transparent electrode, a liquid crystal layer, and a color filter are sandwiched between glass plates, and two polarizing plates are provided on both sides. Each polarizing plate is constituted in such a way that a polarizer (hereinafter also referred to as a polarizing membrane or a polarizing film) is sandwiched between two optical films (namely polarizing plate protective films). As the above polarizing plate protective film, commonly employed is a cellulose triacetate film.
On the other hand, due to recent technical progress, liquid crystal display devices are increasing in size, and simultaneously, application of liquid crystal display devices is diversifying. Examples thereof include an application as a large display installed along streets and at shops as well as an advertisement display in public places employing display equipment called digital signage.
In the above applications, since an outdoor application is assumed, deterioration due to moisture absorption of the polarizing film results in problems, whereby higher moisture resistance of the polarizing plate protective film has been sought. However, it has been difficult to realize sufficient moisture resistance by employing conventional cellulose ester films such as a cellulose acetate film. When the film thickness is increased to realize desired moisture resistance, problems have occurred in which optical effects become great. Further, in recent years, a decrease in thickness of these devices is demanded, whereby any increase in thickness of the polarizing plate has caused problems.
On the other hand, as a material of a low moisture absorptive optical film, polymethyl methacrylate (hereinafter abbreviated as PMMA) has been appropriately employed due to its excellent transparency and dimensional stability, in addition to low hygroscopicity.
However, as described above, along with a large increase of liquid crystal display devices and an increase in their outdoor application, in order to enable sufficient recognition of outdoor images, it is necessary to increase the light amount of backlights and at the same time, they are employed under more severe conditions, whereby heat resistance at higher temperatures and over extended periods has been sought.
However, PMMA film exhibits low heat resistance. When employed at a relatively high temperature over an extended period, problems have occurred in which the film shape is modified.
The above problems have been not only for physical properties of film itself, but also more serious ones for polarizing plates and display devices which employ the above film. Namely, in a liquid crystal display device, the polarizing plate curls along with deformation of the film, whereby problems have occurred in which the entire panel suffers from curling. Problems due to film deformation occur also on the backlight side. When employed in the position of the surface of the visible side, the designed retardation difference is modified, whereby problems occur in which the view angle fluctuates and colors shi.
Further, when an acrylic resin film is compared to cellulose ester films, the acrylic resin film is more fragile and more brittle to result in handling difficulty, whereby it has been difficult to stably manufacture optical films, especially for large-sized liquid crystal display devices.
To overcome the above problems, a method is proposed in which polycarbonate (hereinafter referred to as PC) is added to acrylic resins. Due to limitations in employable solvents and insufficient mutual compatibility of the used resins, while turbidity tends to result, whereby it has been difficult to employ it as an optical film (refer, for example, to Patent Document 1)
As other methods to improve heat resistance, disclosed is a method in which an alicyclic alkyl group is introduced as a copolymerization component of the acrylic resin, as well as a method in which a ring structure is formed in the molecular primary chain via an intramolecular ring forming reaction (refer, for example, to Patent Documents 2 and 3).
These methods improved heat resistance but did not realize film which exhibited targeted brittleness, whereby it was difficult to manufacture an optical film employable in a large-sized liquid crystal display device. Further, the resulting brittleness is not sufficiently low, and the optical film tended to deform the panel, and it was not possible to reduce the variation of retardation, whereby problems of fluctuation of the view angle and color shift occurred.
As a technique to improve moisture resistance and heat resistance, proposed are resins in which acrylic resins are combined with impact resistant acryl rubber-methyl methacrylate copolymers and modified acetyl cellulose (refer, for example, to Patent Document 4).
However, even by employing the above method, the brittleness was not sufficiently lowered as desired, and handling properties were insufficient to manufacture an optical film employed in a large-sized liquid crystal display device. Further, haze due to mixed components was generated and when employed in an outdoor setting where a higher contrast was required, problems occurred in which image contrast was lowered.
In addition, a technique is proposed in which, with regard to conventional cellulose ester film, acrylic resins are blended to control plasticizers and optical characteristics (refer, for example, to Patent Document 5).
However, in these objects, acrylic resins are not added in an amount which sufficiently enhances moisture resistance, whereby sufficient moisture resistance is not realized. Accordingly, under a high humidity ambience, problems have occurred in which polarizing plates deteriorate and optical values of optical films vary. Further, it is commonly assumed that when a large amount of other resins is added to cellulose esters to enhance moisture resistance, transparency is reduced. Accordingly, no cellulose ester film has been produced in which moisture resistance is enhanced so that under a high humidity ambience, optical values remain stable.
Under the above situations, along with recent expansion of application of liquid crystal display devices, problems of employed optical films such as low hygroscopicity, transparency, high heat resistance, or brittleness have increasingly pronounced and improvements have been sought.    (Patent Document 1) Japanese Patent Publication Open to Public Inspection (hereinafter referred to as JP-A) No. 5-306344    (Patent Document 2) JP-A No. 2002-12728    (Patent Document 3) JP-A No. 2005-146084    (Patent Document 4) JP-A No. 5-119217    (Patent Document 5) JP-A No. 2003-12859